The present invention relates to solid-state imaging apparatus for use for example in a video camera, digital still camera, etc., and more particularly relates to the solid-state imaging apparatus using an amplified solid-state imaging device having an amplification function within its imaging region.
In recent years, MOS (Metal Oxide Semiconductor) type imaging devices (image sensor) are drawing attention and are practically used as solid-state imaging device. The MOS imaging devices are capable of being driven by a single power supply as compared to CCD (Charge Coupled Device) type imaging devices. Further, while CCD imaging devices require an exclusive process, the MOS image sensors have an identical manufacturing process as other LSI's and thus are superior in providing many functions as they are readily made into an SOC (system on chip). Furthermore, since the MOS image sensors have an amplification circuit (amplifier) in each individual pixel to amplify signal electric charges within the pixel, they are less likely to be affected by noise due to the transmission path of signals. Moreover, the signal electric charges in each pixel of the MOS image sensor can be extracted by means of a selection system so that in theory, the accumulation time and/or read sequence of signals can be arbitrarily controlled pixel by pixel.
With the MOS image sensors, when a high-luminance light is incident on a partial region, a change in electric potential occurs along a left and right direction of the rows on which the high-luminance light is incident so that an image as if a white band-like light is incident is obtained. Such a phenomenon in the present specification will be referred to hereinafter as “transverse stripe phenomenon” at the time of incidence of high-luminance light. FIG. 1 schematically illustrates an image in the case with an occurrence of the transverse stripe phenomenon at the time of incidence of high-luminance light. Shown in FIG. 1 is an output image when the high-luminance light is incident in the vicinity of a center of the image sensor, where a band-like transverse stripe due to change in electric potential is seen along the row direction on the left and right thereof. This transverse stripe phenomenon at the time of incidence of high-luminance light will be described below together with a typical operation of MOS image sensor.
FIG. 2 shows a block diagram of a typical MOS image sensor. Pixels for converting light into electrical signal are two-dimensionally arranged into a matrix, where P(i, j) represents a unit pixel of i′ th row and j′ th column (i, j: an integral number). The pixel P(i, j) is to output a received light as voltage onto a vertical signal line SL-V(j) by means of a reset pulse φ RS(i), a transfer pulse φ TX(i), and a select pulse φ SE(i) outputted from a vertical scanning circuit 101. The pixel P(i, j) is connected to a constant current supply CCS(j) and to a CDS (Correlated Double Sampling) circuit CDS(j) through the vertical signal line SL-V(j). The CDS circuit CDS(j) is connected to a horizontal signal line SL-H through the vertical signal line SL-V(j). The CDS circuit CDS(j) eliminates variance of signal that is different from one pixel to another by differentiating between a reset signal and an image signal from the pixel (i, j) by means of a clamp pulse φ CL and a sample-and-hold pulse φ SH. An output from the CDS circuit CDS(j) is outputted onto the horizontal signal line SL-H by means of a column select pulse φ HP outputted from the horizontal scanning circuit 102 so as to obtain a signal Vout amplified at an amplification circuit Amp. By repeating the above operation for a predetermined number of pixels, two-dimensional image signals can be obtained. These operations are controlled by signals outputted from the vertical scanning circuit 101 and the horizontal scanning circuit 102 under instructions from a control signal generation circuit 103.
FIG. 3 shows a circuit diagram of the pixel P(i, j) and the constant current supply CCS(j) in FIG. 2. A unit pixel P(i, j) includes: PD(i, j), a photodiode; Mtr(i, j), a transfer transistor; Msf(i, j), an amplification transistor; Mrs(i, j), a reset transistor; Mse(i, j), a select transistor; FD(i, j), a floating diffusion section; SL-V(j), a vertical signal line; Vgs(i, j), a gate-source voltage of the amplification transistor Msf(i, j); CCS(j), a constant current supply; Vccsgs(j), a gate-source voltage of the transistor of the constant current supply CCS(j); Vsl-v(j), a voltage of vertical signal line; SL-V(j), a vertical signal line; Vdd, a power supply voltage line; Vccsg, a gate voltage line of the transistor within the constant current supply CCS(j).
FIG. 4 is a circuit diagram of the CDS circuit CDS(j) in FIG. 2. FIG. 4 includes: Mcl(j), a clamp transistor; Msh(j), a sample-and-hold transistor; Ccl(j), a clamp capacitor; Csh(j), a sample-and-hold capacitor; Vcdso(i, j), an output voltage from the CDS circuit CDS(j); Vcl, a clamp voltage; φ CL, a clamp pulse; and φ SH, a sample-and-hold pulse. At the CDS circuit CDS(j), a reset signal (voltage at the time of attaining φ CL=Low) and a signal of light (voltage at the time of attaining φ SH=Low) of pixel in the column j are differentiated so as to output Vcdso(i, j).
An operation in the case where a high-luminance light is incident in the MOS image sensor having such construction will now be described by way of FIG. 5.    (1): when a light having a large light amount has entered the pixel (i, j), the electrical potential of the vertical signal line SL-V(j) at the time of outputting an image signal of the pixel (i, j) is extremely lowered so that a voltage outside of an operation range of the current supply CCS(j) (voltage in a non-saturation region of the drain-source voltage Vccsds of the transistor of the current supply CCS(j)) is applied on the current supply CCS(j).    (2): as a result that the condition of (1) has been attained, the current supply CCS(j) is caused to operate in its non-saturation region so that the electric current becomes smaller than normal.    (3): as a result that the condition of (2) has been attained, a total sum of the currents following through a ground line GL-H connected to each current supply is also reduced. A voltage drop due to wiring resistance Rgl-h(j+k) on the ground line connected to the current supply CCS(j) is thereby reduced so that the gate-source voltage Vccsgs of the transistor within the current supply CCS(j+k) becomes larger.    (4): as a result that the condition of (3) has been attained, i.e. due to the fact that the gate-source voltage Vccsgs of the transistor within the current supply CCS(j+k) is increased, a current greater than normal is caused to flow through the vertical signal line SL-V(j+k) connected to the pixel P(i, j+k) on which the high-luminance light is not incident.    (5): as a result that the condition of (4) has been attained, when the current is increased, the gate-source voltage Vgs of the amplification transistor within the pixel P(i, j+k) also becomes greater than that at the time of incidence of normal light, and as a result, the value of signal becomes greater than normal (lighter as an image).
Accordingly, a dark region reflected as black on the image when condition is normal or a region on the left and right of the region on which the high-luminance light is incident is reflected on the image inevitably in a lighter-than-black condition when the high-luminance light is incident. As a result, a lighter band-like transverse stripe is observed along a row direction on the left and right of where the high-luminance light is incident.
As a measure to avoid this transverse stripe phenomenon occurring at the time of incidence of high-luminance light, one constructed as in the following has been proposed for example in Japanese Patent Application Laid-Open 2001-230974. Specifically in the proposed construction, a clip circuit for example to clip voltage of the vertical signal line is separately provided to clip the vertical signal line by a predetermined voltage in the period of reading signal of light so that the voltage of the vertical signal line is controlled so as not to depart from an operation range of the current supply.